Microelectromechanical systems (MEMS) microphones that produce pulse density modulated (PDM) (e.g., an M times over-sampled single bit audio data generating digital microphone) in response to sound are becoming a popular replacement for conventional condenser microphones. In addition, many modern devices use audio signal processing algorithms (e.g., encoding and decoding algorithms) having Nyquist rated multi-bit resolution LPCM audio data. Therefore, many modern devices use audio signal processing algorithms (e.g., encoding and decoding algorithms) having Nyquist rated multi-bit resolution linear pulse code modulated (LPCM) audio data. As such, a conversion process is required to convert M times oversampled single bit data (e.g., PDM) to multi-bit Nyquist rated data (LPCM).
However, MEMS microphones (in addition to many other audio devices) are typically driven by a clock having a restricted range of clock frequencies. As such, in many MEMS microphones, the conversion to LPCM data having a Nyquist rate is not possible since MEMS microphones are driven with a clock having a restricted range of clock frequencies. For example, if the clock has a frequency range between 1 MHz to 4 MHz, a 64th order decimation process cannot provide LPCM data having sample rates below approximately 16 KHz and above approximately 64 KHz. As such, common audio processing frequencies (e.g., 8, 11.05, 12, 88.2, 96, 176.4 and 192 kHz) are excluded from LPCM data, which is undesirable.
Accordingly, a need exists for a circuit and method can provide LPCM data having sample rates that are unrestricted by the clock frequency ranges.